1. Field of the Invention
The present invention relates to a system, an apparatus, and a method for cancelling interferences of received signals. More particularly, the present invention relates to a system, an apparatus, and a method for cancelling interferences of received signals according to pilot tones and processed data.
2. Descriptions of the Related Art
Orthogonal frequency division multiplexing (OFDM) is one of common transmission techniques that has been wildly used in wireless communications. In a highly-mobility OFDM system, channels would change rapidly. As the result, the orthogonality between sub-carriers would be destroyed and inter-carrier interferences (ICI) would be induced. A received signal that has been transmitted through this noisy channel may contain erroneous messages as well.
FIG. 1 illustrates a conventional OFDM system, which comprises a transmitter 11 and a receiver 12. The transmitter 11 comprises a source bit generator 111, a signal mapper 112, a serial to parallel converter (S/P) 113, an inverse fast Fourier transform (IFFT) 114, a guard interval adder 115, a parallel to serial converter (P/S) 116, and a transmitting antenna 117. The receiver 12 comprises a receiving antenna 121, an S/P 122, a guard interval remover 123, an FFT 124, an ICI mitigation module 125, a decision mechanism 126, a P/S 127, and a signal demapper 128.
In sequence, the source bit generator 111 first generates a plurality of bits for transmission, and the signal mapper 112 can map the bits generated by the source bit generator 111 into a formatted signal. The S/P 113 converts the formatted signal into an original input parallel signal {tilde over (x)} 101. The IFFT 114 then applies an IFFT operation to the original input signal {tilde over (x)} 101, resulting in an IFFT signal. The guard interval adder 115 adds guard intervals to the IFFT signal to derive a guarded signal for the P/S converter 116 to convert the guarded signal into a serial form. The transmitting antenna 117 then transmits the signal.
In the receiver 12, the receiving antenna 121 receives a serial signal transmitted from the transmitter 11 through the transmission channel 13. The transmission channel 13 is a time-varying multipath channel. The S/P 122 converts the serial signal into a parallel signal. Then, the guard interval remover 123 removes the guard interval to derive an unguarded signal. Thereafter, the FFT 124 applies the FFT operation to the unguarded signal to derive an interfered signal {tilde over (y)} 102. As a note, the ICI mitigation 125 has to cancel the aforementioned ICI within the interfered signal {tilde over (y)} 102 in order to derive the original input signal {tilde over (x)} 101. After that, the decision mechanism 126, the P/S 127, and the signal demapper 128 perform their corresponding operations to derive the original source bits.
FIG. 2 illustrates a conventional SIC block 2 that is able to cancels the ICI. The convention SIC block 2 can play the role of the ICI mitigation block 125 in FIG. 1. The SIC block 2 comprises a minimum mean square error (MMSE) equalizer 211, a hard decision module 212, a channel gain module 213, a multiply module 214, a delay module 215, and an add module 216. This type of approach strongly depends on channel estimation. Its performance is usually not satisfactory because the channel is estimated according to a few pilot tones.
Before going into the details of the SIC block 2, the signal model of an OFDM system is first addressed. The time domain signal model usually presents a signal as:
            y      ⁡              (        k        )              =                            ∑                      l            =            0                                N            -            1                          ⁢                              h            ⁡                          (                              k                ,                l                            )                                ⁢                      x            ⁡                          (                              k                -                l                            )                                          +              z        ⁡                  (          k          )                      ,wherein x(k) denotes the input signal, y(k) denotes the received signal, h(k,l) represents the lth channel tap at time instant k, N represents the number of sub-carriers, and z(k) represents noise. The received signal of the N sub-carriers can be expressed in the vector form as y=Hx+z.
The frequency domain model usually represent the signal {tilde over (y)}={tilde over (H)}{tilde over (x)}+{tilde over (z)}, wherein {tilde over (y)} represents the received model in the frequency domain, {tilde over (x)} represents the input model in the frequency domain, {tilde over (H)} represent the ICI channel matrix, and {tilde over (z)} represents the noise in the frequency domain. Furthermore, {tilde over (H)}=GHGH, wherein G represents a fast Fourier transform (FFT) matrix.
One of the tasks of the receiver 12 is to recover the signal {tilde over (x)} 101 from the interfered {tilde over (y)} 102. The SIC block 2 successively cancels the interference to achieve that. First, {S1, S2, . . . , SN} are ordered according to their power in a decreasing manner, wherein each of the Si corresponds to a column of {tilde over (H)}. Then, the SIC block 2 performs successive ICI cancellations according to {S1, S2, . . . , SN} in order.
The function of the MMSE equalizer 211 is to minimize the cost function, E[∥{tilde over (x)}−QHy∥2]. Thus, the MMSE equalizer 211 is derived the following way:
      Q    =                            (                                                                      H                  ~                                H                            ⁢                              H                ~                                      +                                                            σ                  Z                  2                                                  σ                  X                  2                                            ⁢                              I                                  N                  ×                  N                                                              )                          -          1                    ⁢                        H          ~                H              ,wherein {tilde over (H)} represented the estimated channel. The estimation of {tilde over (H)} is then substituted into the MMSE equalizer.
To be more specific, the interfered signal {tilde over (y)} 102 is inputted both into the MMSE equalizer 211 and the delay module 215. In the order {S1, S2, . . . , SN}, the SIC block 2 processes the signal related to Si in sequence. The MMSE equalizer 211 generates an equalized signal 201, xSi=qSiHyi-1, corresponding to Si. The equalized signal 201 is then inputted to the hard decision module 212 to derive a decided signal 202, {tilde over (d)}Si. The channel gain {tilde over (h)}Si 203 corresponding to the decided signal 202 is provided. The multiply module 214 multiplies the decided signal 202 and the channel gain {tilde over (h)}Si 203 to derive a reconstructed signal 204. The delay module 215 delays the interfered signal {tilde over (y)} 102 for a predetermined length of time to derive a delayed signal 205. Then, the add module 216 adds the delayed signal 205 and the negative of the reconstructed signal to remove the interference and get an output signal {tilde over (y)}i={tilde over (y)}i-1−{tilde over (h)}Si{tilde over (d)}Si.
However, the performance of this type of method relies highly on the accuracy of channel estimation. Due to the limited number of pilot tones, the channel response cannot be accurately estimated. Thus, the performance is unsatisfactory most of the time. Consequently, a system, an apparatus, and a method that cancels ICI accurately are still critical issues in this field.